A common way in which power at low voltage is furnished in locations where the load power density is high and where such power is to be relatively uninterruptable is by the use of low-voltage grid or spot networks which are supplied with power from more than one set of high-voltage feeders through voltage reducing transformers. The number of such high-voltage feeders which supply the low-voltage network will of course depend upon the magnitude of the load which is to be supplied by the network as well as the level of reliability or security desired for the network.
Such feeders supply the low-voltage network through network transformers which have their secondary or low-voltage windings connected to the low-voltage network through a three-phase circuit breaker. These circuit breakers which are controlled by network protector relays are often located in vaults in the street and it is desired that the opening and closing thereof be automatic. When, for example, it is desired to work on the high-voltage feeder and the feeder is de-energized at the sub-station, the three phase circuit breaker controlled by the network protector relay should open. When the feeder is again energized at the sub-station the circuit breaker should automatically close providing however, that the phase rotation on both sides of the circuit breaker is the same, that the phase relationship between the voltage at the output of the feeder transformer and that in the network is within the proper limits, and that the magnitude of the network transformer output voltage relative to that in the network is proper so that when the circuit breaker is closed power will flow into the network.
A network protector relay designed to open the circuit breaker when the positive-sequence power flows outwardly of the low-voltage network, as for example that which is required by the magnetizing energy of the network transformer and which will close upon proper reenergization of the feeder is shown and described in U.S. Pat. No. 3,248,609 dated Apr. 26, 1966 and assigned to the same assignee as is this application.
Under certain fault conditions in the feeder, such as for example a single line-to-ground fault, the flow of positive-sequence power will remain in a direction from the feeder to the network. Under these conditions the network protector relay described in the above identified U.S. Patent would not respond to the fault condition until after the sub-station breaker opened to deenergize the feeder and the flow of positive-sequence power is then flowed outwardly of the low-voltage network. This interval could take upwardly of eight cycles which in the case of a 60 HZ line would amount upwardly of 130 milliseconds. If the network protector relay were capable of detecting the faulted feeder, the three-phase breaker connecting the grid or spot network to the transformer could be opened in a shorter time period with less damage. If fuses were used in the primary feeder which supplies the network transformer they could blow as well as those of other primary feeder lines and, depending upon the number of primary feeders supplying the network, the result could be low-voltage network shutdown.